The commercial production of polyurethane via isocyanate polyaddition reactions requires the use of catalysts. Tertiary amines were widely accepted in the industry as versatile polyurethane catalysts. They are generally stable in the presence of standard polyurethane formulation components and can impact both the blowing (water-isocyanate) and gelling (polyol-isocyanate) reactions. Unfortunately, a number of commonly used tertiary amine catalysts are relatively volatile and have an unpleasant smell. Even at low use levels, they may impart a noticeably pungent odor to the polyurethane formulation. It would be desirable to identify alternatives to standard tertiary amine catalysts which are not malodorous, yet exhibit the same type of activity in isocyanate polyaddition reactions.
One strategy for the reduction of odor associated with tertiary amine catalysts is the identification of less volatile structures. The literature teaches numerous techniques for reducing volatility, particularly increasing molecular weight or improving hydrogen bonding capability.
U.S. Pat. No. 4,026,840 describes the use of hydroxy-functionalized tertiary amines as polyurethane catalysts, particularly useful for the production of polyisocyanurate. In addition to lower volatility, such materials also contain reactive functionality which bind the amine into the final product. Typically, however, catalysts which are non-fugitive by way of reaction with isocyanate must be used at relatively high levels to compensate for immobilization of the tertiary amine functionality during urethane part production.
U.S. Pat. No. 4,006,124 which is an example of lower volatility being achieved by metal complex formation, describes the use of amidine-metal complexes as polyurethane catalysts.
U.S. Pat. No. 4,857,560 mentions reduced odor emission as an additional advantage of the disclosed expansion catalysts formed from tertiary amines and an acid mixture of (1) boric acid and (2) a carboxylic acid. Unfortunately, strong organic acid containing catalysts tend to show masterbatch instability and corrosivity. Furthermore, carboxylic acid functionality is reactive with and can consume expensive isocyanate functionality, resulting in the formation of amide rather than the more standard urethane or urea segments in a resulting part.
Boric acid derivatives have not otherwise been used in combination with tertiary amines to effect amine odor reduction. Other uses of boric acid in polyurethane formulations, however, have been described.
CA 99(6):39229a notes that boric acid, when used as a filler at 10-40%, decelerates foaming in polyurethanes based on Laprol 805 and Lapramol 294 polyether polyols, polyisocyanate, Freon 113 and water.
"Int. Prog. Urethanes 1980", 2, 153-73 describes the use of boric acid as a blowing agent, its behavior being almost equal to that of water. Neither reference describes the effect of boric acid at use levels typified by a catalyst, nor do they indicate the effect of boric acid on polyurethane catalysis in the presence of a tertiary amine.
U.S. Pat. No. 4,611,013 describes the use of quarternary ammonium borates to effect the concurrent trimerization/carbodiimidization of polyisocyanates. The borates are prepared from boric acid, alcohols and quarternary ammonium hydroxides and as such are not derived from tertiary amines. Other examples are given by U.S. Pat. Nos. 4,530,938 and 4,425,444.
U.S. Pat. No. 3,193,515; U.S. Pat. No. 3,127,404 and FR 2,301,554 disclose the use of boric acid in the preparation of an ammonium salt polyurethane catalyst from triethylenediamine and a glycol borate acid. Glycol borate acids are prepared by heating mixtures containing not substantially less than 0.5 moles of boric acid per mole glycerol or alternative vicinal glycol to effect acid complex formation. The advantage of such catalyst composition is delayed activity and/or accelerated cure. Unfortunately, the preferred catalyst compositions are highly viscous because a low glycol/boric acid ratio is required to minimize the amount of glycol delivered into a formulation. The addition of low molecular weight glycol into polyurethane parts, particularly foam parts, is typically avoided to prevent unnecessary consumption of expensive isocyanate. Further, glycols can negatively impact physical properties.
"J. Org. Chem." 1972, 37 (14), 2232 discloses that monodentate nitrogen nucleophiles do not react significantly with boric acid in aqueous solution.